1. Field of the Invention
The present invention relates to signal interface circuits, and in particular, to signal interface circuits in which specific circuit parameters and performance characteristics, such as signal gain and terminal impedance, can be selectively controlled.
2. Description of the Related Art
Conventional signal interface circuits exist in many forms and configurations for performing many different functions. One function often performed by such circuits is that of impedance matching whereby two circuits or systems having different impedances can be interconnected so as to provide for maximum power transfer or signal gain. However, due to the inherent frequency dependency of all impedances, the impedance matching, power transfer and signal gain characteristics are dependent upon the frequencies of the signals being conveyed by such interface circuits and can, therefore, vary significantly. Further, attempts to optimize these performance parameters, i.e. establishing and maintaining the desired frequency response characteristics for these performance parameters, can be quite difficult since adjustment of one will generally affect the others as well.
This problem becomes further complicated when the interface circuit must pass signals in two directions. For example, the interface circuit may be required to provide impedance matching and signal gain adjustment simultaneously for transmit and receive circuits which are connected to and must send bidirectional signals through a shared signal node. One example of a circuit requiring such two-way impedance matching and frequency compensation is a subscriber line interface circuit used in telecommunications. Such an interface circuit must provide a complex impedance termination and matching transmit and receive frequency response characteristics to the subscriber line which is attached at the "tip" and "ring" terminals and which itself has a complex impedance associated therewith.
In conventional interface circuits of this type, optimization or adjustment of multiple performance parameters is typically done by providing for the adjustment or design of circuit components or groups Of circuit components which correspond to each individual performance parameter. For example, one set of circuit components would be designed or adjusted to provide for a desired complex impedance termination, while other circuit components would be designed or adjusted to provide for the desired transmit or receive signal frequency responses. Accordingly, matching all three performance parameters can be quite difficult.
This problem of matching multiple performance parameters is further exacerbated when manufacturing tolerances, temperature and other effects are taken into consideration. The circuit component or components designed or adjusted to provide these desired performance parameters must often be matched to one another and will have multiple parameter "bandwidths", i.e. a "bandwidth" associated with each operating parameter such as temperature, etc. Therefore, not only must each performance parameter be matched, but they must remain matched as various operating parameters, such as temperature, vary.
Accordingly, it would be desirable to have a signal interface circuit with selectable signal interface parameters which simplify the providing and maintaining of desired impedance and signal frequency response characteristics.